1. Field of the Invention
The present invention relates to a hollow golf club head.
2. Description of the Related Art
As wood golf club heads such as driver and fairway wood golf club heads, hollow metallic golf club heads are well known. In general, a hollow wood golf club head has a face portion for hitting a golf ball, a crown portion which forms the upper surface portion of the golf club head, a sole portion which forms the bottom surface portion of the golf club head, a side portion which forms the toe-, back-, and heel-side side surface portions of the golf club head, and a hosel portion. A shaft is inserted into the hosel portion, and fixed by, for example, an adhesive.
Although an aluminum alloy, stainless steel, or a titanium alloy is often used as a material which constitutes the hollow golf club head, titanium alloy in particular is extensively used these days.
Japanese Patent Laid-Open No. 2009-153802 describes a golf club head having a thin portion at the central portion of the crown portion, and a thick portion surrounding it. The thin portion is formed by casting and chemical milling. In this golf club head, portions other than the face portion are integrally molded by casting, and a bulged portion is formed in the crown portion to bulge outwards. After casting, the protrusion of the bulged portion is removed by chemical milling to form a thin portion.
In Japanese Patent Laid-Open No. 2009-153802 mentioned above, only thin and thick portions are formed in the crown portion. With such a structure, a step between the thin and thick portions is large, so stress is likely to concentrate at the boundary portion between the thin and thick portions upon striking a golf ball.
To solve this problem, a medium thick portion can be formed between the thin and thick portions, and a cavity-shaped mold for casting, which has a step portion for forming the medium thick portion, is used in this case. It is thus necessary to use a mold with another shape every time the width and thickness of the medium thick portion are changed, thus increasing the cost of the molds.
As in Japanese Patent Laid-Open No. 2009-153802 mentioned above, when the protrusion of the bulged portion is removed to form a thin portion, a trace of removal of the protrusion of the bulged portion remains in the crown portion. Hence, the boundary between the removed bulged portion and the unremoved portion surrounding it becomes conspicuous, and degrades the aesthetic appearance.